1. Field of the Invention
The invention relates to equipment for handling cylindrical metal containers. More specifically, the invention is an improved apparatus for carrying and rotating metal can bodies having an open end and a closed end through an ultra violet curing oven.
2. Description of the Prior Art
With the development of inks that cure with ultra violet light, it became possible to apply decorative coatings to numerous types of containers with high speed, and to cure the coatings without having to evaporate solvents, which present an air pollution problem. Empirical studies indicated that the best and most rapid curing of ultra violet curing inks occurs when the ink is repeatedly exposed to UV radiation. Accordingly, a sheet of newspaper with UV curing inks may, for example, be passed under a plurality of transversely aligned UV lights to cure the inks through repeated exposure to radiation.
In the container arts, the first known curing apparatus for cylindrical containers having UV curing inks on their exterior wall utilized a simple belt conveyor on which the containers were placed endwise. The width of the belt was slightly narrower than the diameter of the container end so that the lip of the container also contacted a stationary support area adjacent the belt. As the belt moved the container linearly, the lip of the container end dragged against the stationary support area and induced axial rotation in the container. Printing machines using this type of belt system for curing inks were known to operate at curing speeds as high as three hundred units per minute. The problems of such a system are that each container must be balanced endwise on the belt, and the belt speed is limited by the need to keep the container balanced on its end as it travels through the UV oven.
Another attempt in the prior art is represented by the apparatus of U.S. Pat. No. 3,840,999 to Whelan. This apparatus used a well known and established can handling technique of mounting each container on a mandrel or brush, such as is commonly done in can decorators to both transport the can body and rotate the can body for printing. One problem with this type of machine is that mounting a can on a mandrel or brush is a fairly precise operation, and in can printers the mechanisms for accomplishing this task are the subject of a number of patents. Another problem is that in can printers the path of the can is limited to a simple circle in most cases, while the path through a UV curing oven is preferred to be a linear path of substantial length. Thus, a wheel holding the mandrels in predetermined positions for can loading is not a practical structure, while a linear conveyor that can travel through an oven is a practical structure but is extremely difficult to load with preciseness. A further problem is that unloading a brush or mandrel on a linear chain is quite difficult to accomplish without marring the can exterior. In can printers, the can is "blown off" the mandrel with air, but air manifolds are not easily connected to a moving roller chain, as they might be to a can carrying wheel in a printer.
U.S. Pat. No. 3,894,237 teaches a novel idea of mounting a can on a peg chain for rotation through an ultra violet curing oven. This structure offers the advantage of easy loading, as the pegs shown in the patent are substantially narrower than the inner diameter of the can and resemble the pins of well known pin-chain conveyors, as are commonly used in the art to move decorated cans away from a printing machine for drying of conventional solvent based inks. Similarly, the cans may be unloaded from the pegs with proven can handling equipment, as is well known in the prior art for use with pin-chain conveyors.
The problems of a peg chain conveyor are primarily ones of stability, since the cans must rotate on the peg in order or expose all sides of the cans to UV radiation. When a peg in a can first rotates, there is considerable instability as the surface of the peg attempts to drive the inner surface of the can essentially along a line of tangential contact. If the friction between the can and peg is too great, the peg will spin the can around the axis of the peg in highly unstable condition. Furthermore, the peg must contact the inside of the can only against the wall of the can and not against the bottom of the can, as unstability results if any rotational force upsets the rotational motion of the can around its own axis. Another problem is that friction between the peg and the can must be controlled to allow smooth and even rotation of the peg in the can, without sudden grabbing of the peg against the can wall, as has been found to cause a tossing or bouncing of the can on the peg.
A problem that can occur in any can carrying device but that is especially of concern in peg-chain style conveyor is that a can may not be fully mounted on the peg, and the extending portion of the can may contact a stationary part of the UV oven and bend or break the peg. Replacing a broken peg requires that the peg-chain be stopped, the central pin of the peg be forced out of the chain, and the new pin installed. This process can result in damage to the chain, as the pin is typically serving also as a hinge pin connecting the links of the chain. Thus, repair of a bent peg means separating the chain and may also result in some bending damage to the side plates of the chain links hinged on the pin.
The above problems, as well as others, are overcome in the present invention, which is an improvement over the rotating means disclosed in U.S. Pat. No. 3,894,237.